Light emitting diodes (LEDs) and in particular high intensity and medium intensity LED strings are rapidly coming into wide use for lighting applications. LEDs with an overall high luminance are useful in a number of lighting applications including, but not limited to, backlighting for liquid crystal display (LCD) based monitors and televisions, collectively hereinafter referred to as a monitor. In a large LCD monitor the LEDs are typically supplied in one or more strings of serially connected LEDs, thus sharing a common current. Similarly, in general lighting applications, the LEDs are typically supplied as one or more strings of serially connected LEDs sharing a common current.
In order supply a white light, for a backlight of a monitor or for general lighting, one of two basic techniques are commonly used. In a first technique one or more strings of “white” LEDs are utilized, the white LEDs typically comprising a blue LED with a phosphor which absorbs the blue light emitted by the LED and emits a white light. In a second technique one or more individual strings of colored LEDs are placed in proximity so that in combination their light is seen as a white light. Often, two strings of green LEDs are utilized to balance one string each of red and blue LEDs. In the case of colored LEDs, a further mixer is required, which may be part of the diffuser, to ensure that the light of the colored LEDs are not viewed separately, but are rather mixed to give a white light. The white point of the light is an important factor to control, and much effort in design and manufacturing is centered on the need for a controlled white point.
In certain applications, each of the colored LED strings is typically controlled by both amplitude modulation (AM) and pulse width modulation (PWM) to achieve an overall fixed perceived luminance and color balance. AM is typically used to set the white point produced by the disparate colored LED strings by setting the constant current flow through the LED strings to a value determined as part of a white point calibration process and PWM is typically used to variably control the overall luminance, or brightness, of the monitor without affecting the white point balance. Thus the current, when pulsed on, is held constant to maintain the white point produced by the combination of disparate colored LED strings, and the PWM duty cycle is controlled to dim or brighten the overall luminance by adjusting the average current over time. The PWM duty cycle of each color is further modified to maintain the white point, preferably responsive to a color sensor. It is to be noted that different colored LEDs age, or reduce their luminance as a function of current, at different rates and thus the PWM duty cycle of each color must be modified over time to maintain the white point.
Each of the disparate colored LED strings has a voltage requirement associated with the forward voltage drop of the LEDs and the number of LEDs in the LED string. In the event that multiple LED strings of any particular color are used, the voltage drop across strings of the same color having the same number of LEDs per string may also vary due to manufacturing tolerances and temperature differences. Ideally, separate power sources are supplied for each LED string, the power sources being adapted to adjust their voltage output to be in line with the voltage drop across the associated LED string. Such a large plurality of power sources effectively minimizes excess power dissipation however the requirement for a large plurality of power sources is costly.
An alternative solution, which reduces the number of power sources required, is to supply a single power source for each color. Thus a plurality of LED strings of a single color is driven by a single power source, and the number of power sources required is reduced to the number of different colors, i.e. in the case of colored LEDs typically to 3. Unfortunately, since as indicated above different LED strings of the same color may exhibit different voltage drops, such a solution further requires an active element in series with each LED string, also known as a dissipative element, to compensate for the different voltage drops so as to ensure an essentially equal current through each of the LED strings of the same color.
FIG. 1A illustrates a high level block diagram of a system for powering and controlling a plurality of LED strings using a single controllable power source according to the prior art. The system includes a controllable power source 11 responsive to a controller 10 implemented as an integrated circuit, a plurality of load resistors RA, RB, RC and RD and a plurality of LED strings 31A, 32A, 33A and 34A. Controller 10 comprises dissipative active elements 20A, 20B, 20C and 20D integrated within the architecture of controller 10. An output of controllable power source 11 is connected in parallel to a first end of each of LED strings 31A, 32A, 33A and 34A, and a second end of each of LED strings 31A, 32A, 33A and 34A is connected to a first contact of a respective one of dissipative active elements 20A, 20B, 20C and 20D. A second contact of each of dissipative active elements 20A, 20B, 20C and 20D is connected to a common potential, such as a ground potential, via a respective one of sense resistors RA, RB, RC and RD.
In operation, power is driven through each LED string 31A, 32A, 33A and 34A by controllable power source 11, with the output voltage level of controllable power source 11 being responsive to an output of controller 10. Controller 10 is further operative to control dissipative active elements 20A, 20B, 20C and 20D so as to ensure that the same level of current is driven through each of LED strings 31A, 32A, 33A and 34A. In particular, the resistance of each of dissipative active elements 20A, 20B, 20C and 20D is controlled, so that the resultant voltage drop across each of sense resistors RA, RB, RC and RD is nearly identical. Furthermore, the voltage drop across each of sense resistors RA, RB, RC and RD is monitored by controller 10 so as to ensure a balanced current, and to determine an appropriate voltage output for controllable power source 11. Dissipative active elements 20A, 20B, 20C and 20D generate heat while dissipating power, negatively impacting the overall die temperature of controller 10. In order to minimize cost, a small die size is preferred for controller 10, and thus there is a severe limitation on the amount of power which may be dissipated by dissipative active elements 20A, 20B, 20C and 20D due to their being packaged within controller 10.
FIG. 1B illustrates a high level block diagram of an alternative arrangement for powering and controlling a plurality of LED strings using a single controllable power source according to the prior art, in which the dissipative elements are not contained within the controller. The system includes a controllable power source 11 responsive to a controller 50 implemented as an integrated circuit, dissipative active elements 40A, 40B, 40C and 40D, a plurality of sense resistors RA, RB, RC and RD and a plurality of LED strings 31A, 32A, 33A and 34A. An output of controllable power source 11 is connected in parallel to a first end of each of LED strings 31A, 32A, 33A and 34A, and a second end of each of LED strings 31A, 32A, 33A and 34A is connected to a first contact of a respective one of dissipative active elements 40A, 40B, 40C and 40D. A second contact of each of dissipative active elements 40A, 40B, 40C and 40D is connected to a common potential, such as a ground potential, via a respective one of sense resistors RA, RB, RC and RD. A control input of each of dissipative active elements 40A, 40B, 40C and 40D is connected to a respective output of controller 50.
In operation, the arrangement of FIG. 1B operates in all respects similarly to the arrangement of FIG. 1A, however placing dissipative active elements 40A, 40B, 40C and 40D external of controller 50 resolves the problem of overheating controller 10. However, the need for control and measurement of the dissipative active elements 40A, 40B, 40C and 40D, and the need to read the voltage drop across each of the external sense resistors RA, RB, RC and RD adds to the total pin count for controller 50, increasing cost.
In summary, neither the arrangement of FIG. 1A, nor the arrangement of FIG. 1B provides an optimum solution taking into account the need for a low footprint, thermal requirements and the need to control the number of pins for a controller.